1. Field of the Invention
This invention relates to ethylene polymer compositions, processes for making such polymer compositions, and molded articles made from such compositions.
2. Description of Related Art
Ethylene homopolymers and interpolymers are known classes of thermoplastic polymers, each having many members. They are prepared by homopolymerizing ethylene or interpolymerizing (e.g., copolymerizing) ethylene with one or more vinyl- or diene-based comonomers, e.g., .alpha.-olefins of 3 to about 20 carbon atoms, vinyl esters, vinyl acids, styrene-based monomers, monomers containing two or more sites of ethylenic unsaturation, etc., using known copolymerization reactions and conditions.
Although ethylene homopolymers and interpolymers are known for their strength and processability, experiments continue to be conducted which explore the outer limits of these properties. One result of this research was the development of substantially linear ethylene polymers (SLEPs) which are less susceptible to melt fracture or formation of extrudate defects during high pressure, high speed extrusion than are conventional polyethylenes. SLEPs possess high processibility and strength without requiring additives.
Thermoplastic elastomers are known materials possessing both elastomeric and thermoplastic characteristics. These materials are becoming increasingly popular in industrial applications because of their ease of fabrication and general good physical properties. Thermoplastic elastomers are of two main types, (1) block and graft copolymers containing soft and hard polymer chain segments, and (2) blends of certain elastomers and thermoplastics. Examples of the latter class include blends of ethylene/propylene rubber (EDPM) with polyolefin thermoplastics, such as polypropylene or polyethylene.
Although the various types of ethylene polymers have found application in a wide range of environments, in certain of these environments improvement is both possible and desirable. For example, applications requiring good performance at elevated temperatures (i.e., temperatures above room temperature) abound. Relevant properties under these conditions include softening point under load, toughness, 100% modulus of elasticity, and compression set. Processability improvements are also desirable, e.g. certain ethylene polymers require a long cooling cycle time in injection molding applications. The need for thermoplastic elastomers with increased high temperature performance that are easy to process is expressed, for example, in U.S. Pat. No. 5,371,143. Polymers or polymer blends which possess a high softening point under load, greater toughness, higher 100% modulus of elasticity, lower compression set, and reduced cycle time are thus desired.